This blog discusses current weather, weather prediction, climate issues, and other topics

Monday, January 6, 2014

Does the Cold Wave Imply Anything About Global Warming? The Answer is Clearly No.

The central and eastern U.S. is now in the midst of a major cold wave, with large regions dropping below zero F and wind chills reaching below -30F. That is fact.

The problem is that the media, some non-governmental organizations, and plenty of individuals are making claims that this event has some kind of implication regarding anthropogenic global warming.

On one hand, some global warming skeptics suggest that such cold is clear evidence that global warming is nonsense.

On the other, global warming "advocates" explain the cold wave as another example of extreme weather forced by increased greenhouse gases.

The truth? Both are wrong.

This individual event says nothing about the impacts of global warming.

Let me give you a sample of these unsupportable claims.

This morning I was listening to the NPR Program "The TakeAway" on which Gary Yohe, professor of Economics and Environmental Studies at Wesleyan University, explicitly stated that the cold wave was the work of global warming.

And Climate Central was pushing the same scary story:

According to Time Magazine:

"Global
warming is sometimes thought of more as “global weirding,” with all manner of
complex disruptions occurring over time. This week’s events show that climate
change is almost certainly screwing with weather patterns ways that go beyond
mere increases in temperature—meaning that you’d be smart to hold onto those
winter coats for a while longer."

And then on the other side, FOX News suggested that the cold wave indicated global cooling.

And hot air expert Donald Trump joined the fray with a well-constructed twitter message:

“This very expensive GLOBAL WARMING ***** has got to stop. Our planet
is freezing, record low temps, and our GW scientists are stuck in ice.” (***** is an expletive that have been removed--this is a family friendly blog)

I could give you a dozen more samples on both sides of this issue. All wrong.

One thing we do know is that the proximate cause of the current cold wave is a substantial undulation of the jet stream, with a deep trough over the central U.S. (see upper level map at 4 AM this morning). Ridges are associated with sinking motion and warming, while troughs are regions of low-level cooling with movement of cold air from the north on their western side. You will notice that the western U.S. has a ridge, with above normal temperatures and a LACK of storminess and snow!

Would we expect global warming to produce such a pattern? More on that later.

Let's examine the claims of both sides of this extreme argument.1. Global warming causes more extreme weather and thus cold waves.

At first glance, you would not give much credence to this suggestion. If temperatures were warming due to mankind's enhancement of greenhouse gases you would expect LESS cold waves.

But a few researchers came up with a hypothesis that as the poles warm the jet stream would weaken and that would result in more waviness of the jet stream. However, recent work by Professor Elizabeth Barnes of Colorado State has shown that there were methodological deficiencies in the research suggesting increased jet undulations with global warming. And that there is no observational evidence of increased waviness. Furthermore, there is little evidence that the jet approaching North America has weakened recently.

"There is likely to be a decline in the frequency of cold air outbreaks
(i.e., periods of extreme cold lasting from several days to over a week)
in NH winter in most areas."

The National Climatic Data Center has a page where you can plot the % of the country with a extreme low minimum temperatures (blue bars). The percentages are declining, which does NOT suggest cold waves are getting more extensive.

The bottom line: the claims that greenhouse warming causes more cold waves like we have seen this week really seems to be without any basis in observational evidence or in theory. The media needs to stop pushing this unsupported argument.

2. The current cold wave proves global warming is not occurring

This claims is really without any basis. Global warming will occur over the the coming century and only can be determined statistically since there is a lot of natural variability in the climate/weather system. One event proves nothing. Furthermore, the real warming is in the future. As shown above, the undulations of the jet stream cause regional changes in temperature and thus there are both warm and cool areas. So if you want to say that global warming is causing the cold and snow in the east, you would also have to say it is causing the warmth and poor-snow conditions in the West. So by their logic, Californians would say "global warming" is real, while easterners would claim the opposite. Gets kind of silly.

Just to illustrate that extremes are NOT increasing in the U.S., NOAA maintains a Climate Extreme Index (CEI) that includes many parameters. Here is a plot of that index for winter for the last century for North America. No trend.

I have a request for you, though... I am a 4th grade teacher, and I link your blog on our class webpage. We love your site! Please blank out swear words, though. I'd hate to have to remove your link from our webpage. The kids love it, and we have many great discussions.

Cliff - your post makes a lot of sense (despite its criticism of my story, which I must point out, notes that many scientists are skeptical of the Arctic warming links)... until you say categorically that the weather has not become more extreme.

Using the CEI as evidence for that, and leaving it there, is incomplete, to say the least. It conflicts with the IPCC AR5, which noted detectable trends in some extremes, and many studies indicating increased heat wave severity, heavy precip events, etc.

Andrew...there is no evidence that I know of, including in the latest IPCC report, suggesting that winter weather in the U.S. has become more extreme ALREADY due to anthropogenic greenhouse warming. Can you cite any? The headline in your article was VERY deceptive. You could have also said that the cold wave MAY be evidence of alien visitation..cliff

Nice post Cliff. Another media craze is blaming the so-called polar vortex on the cold wave...as if we need such an explanation. For example, here is an unfortunate piece from CBS: http://www.cbsnews.com/videos/what-is-the-polar-vortex-and-what-causes-it/

What worries me is that the media's hyping of individual weather events will desensitize people to the real problem of AGW, whose effects may not actually be fully realized until the latter part of this century.

Somebody needs to find a civilized way to begin the discussion about AGW, at which point we as a society can start figuring out how we can curb it.

I would note that your two cases are not equal in their invalidity... if I can use that messy language.

That is to say, those claiming the cold is proof positive against global warming/climate change are clearly wrong, baseless, and really outright lying.

Where as those that are claiming the cold may be caused by unusual perturbations in the jet stream caused by lessening differences between high and mid latitude temperatures are doing so based on citable research. Granted, the paper you cited goes against those conclusions... but then, that's how science works isn't it?

Perhaps what you should be pointing out is that this is an example of where the climate change/global warming theory itself is solid, undeniable and unequivocal. The globe is warming, no matter what brief cold snaps occur from time to time. So the fake sceptics are just that. Fake.

Where as there is open and active and vigorous research ongoing that may link these events to global warming/climate change but the theories are not proven and still very much in need of full observational testing.

Would that not be a more complete, if perhaps a slightly more nuanced position than the "THEY ARE BOTH WRONG" option?

Back in the late Jurassic (when i was a lad) 'The News' didn't make sponsor money (they had sponsors, but they were considered a loss relative to the 8 o'clock line-up). Now, as we all know, news is monetized entertainment, and reporting that some event has less significance than the competitor is boring and presumed to lose eyeballs, hence money. Your thesis here, Dr Mass, probably suffers from being correct, thus boring thus less likely to be heard (-sigh-). perhaps if you wore a flamboyant hat and shouted? but no... i see one of your other commenters wants you to scrub the 'colorful' language, gawdammit.

If the AGW debate were decided by single events there'd be a serious conflict here because Buenes Aires has just had their worst heat wave in several decades. How often our news forgets that there are other hemispheres than ours! Thank you, as always Cliff for being a venerable voice of reason.

Excellent post. As a non-meteorologist, one thing that popped out at me in the CEI chart was what appears to be a 15-20 year cycle in the extreme peaks. I'd guess that this might be something well understood and worth a future blog post about (perhaps it's the PDO?).

In any case, 15-20 years is well beyond what most people have as detailed memory (heck, I can't even remember what I ate for dinner two nights ago), so all such extreme events seem unprecedented every time they show up.

I know this because if any of you thought global warming were true, you’d stop driving, stop flying across the country for the holidays, and stop making excuses for doing so.

Much like the emperor with his new clothes, everyone claims to see the fine threads, but when it comes time to put up, people do what is expedient for themselves. At least until it’s time to make a fuss about global warming again.

The majority of people who champion the global warming cause are affluent. The aren’t big bus riders, and they tend to be more frequent travelers.

If the proponents of global warming stopped warming the globe, we might not need the other people to do it.

Stop trying to convince everyone that it exists, and just do your part already. I’m talking really do your part, not purchasing silly carbon offsets to make yourself feel better. Stop driving. Stop traveling. Stop talking to everyone else about global warming. If it’s real, stop waiting for everyone else to do what needs to be done to curtail it.

Lead by example, and don’t do it in a half-hearted, excuse-laden manner. If New York City is facing extinction in the near future, surely you can forgo flying home for the holidays to save it, right?

People are sick of hypocrisy. They’re tired of “do as I say, not as I do”.

Ending global warming is not going to happen by joining social cause after social cause, and endless finger-wagging at the non-believers.

Frankly, once you start living what you’re all preaching, I’ll be far, far more likely to believe it.

If you don’t care enough about your own cause to make drastic sacrifices, why should anyone else. Seattle is one of the greenest cities in the country, with one of the highest percentages of global-warming proponents, yet the I-5 is clogged each every day.

When that stops being the case, I’ll be glad to take the whole thing more seriously.

Until then, it’s just another way for affluent white people to keep themselves occupied and feel superior while doing it.

Barnes paper: Her claim is that our results (in Francis and Vavrus, Evidence linking Arctic amplification with extreme weather in mid-latitudes, GRL 2012), which were based on an analysis of a particular 500 hPa height contour, was flawed because we did not choose the “waviest” height contour. We chose the contour that best matches the strongest poleward height gradient (varied by season), and thus best matches the path of the jet stream. The waviest contour, however, occurs about 15 deg of latitude farther north where the flow is much weaker, so it is no surprise that the flow is wavier, but more important, the shape of the waves in the flow there is substantially different from that of the jet stream. Using our contour, we *and she* find that in fall (OND), when the Arctic amplification is strongest, the large (Rossby) waves increase in amplitude with time and also progress eastward more slowly, results that support our expectations for the response of the jet stream to Arctic amplification. I therefore submit that her claim is false.

Second, the highly amplified jet-stream pattern existing in recent days and even weeks – and which is responsible not only for the cold spell in the eastern US but also for unusual warmth in AK and Scandinavia, drought in CA, and an extremely stormy period in the UK -- is an excellent example of the sort of pattern we expect to see occur more frequently as Arctic amplification intensifies. Results from several recent papers suggest it’s already occurring (list available on request). Clearly one event cannot be attributed to Arctic amplification or to anything else, but it certainly fits the expectation.

As you can see from some of the more extreme comments on here, this is clearly a 'hot button' issue for many people.

With regard to the psychological elements behind it - what you are seeing is termed 'confirmation bias', whereby people favor information that confirms their beliefs or hypotheses.

The sad truth is, when people have made up their mind about man-made climate change, one way or the other, there will always be plenty of 'data' that they can use to confirm their view. Data that does not confirm their view is simply ignored or ridiculed.

For that reason, I think we are already past the time where we can have a reasoned and rational debate on the issue, particularly in the US.

While John Davis makes some valid points ~ who doesn't cringe when reading of global warming conferences wherein attendees fly in from hither & yon? ~ his argument is ultimately an ad hominem attack. Almost have to wonder if he was intentionally relying on the tu quoque fallacy as he posted.

Thanks for the scientific analysis. Although you are a scientist and report on facts I think it would be responsible for you to add a disclaimer that just because there is no evidence for something specifically is not a reason to do nothing at all.

In the UK we take the stance that regardless of whether Anthropogenic Climate Change is real or not we would rather take a 'pro-active' approach and "do something" than nothing at all. Certainly we aren't still having this elementary debate on whether "peter killed paul or paul killed peter first" :/

By appearing to dismiss such extreme weather events one encourages complacency. Explain this to those who live on the west coast of England for example who had their houses destroyed due to the highest tides in 17 years, perhaps you can explain this?? I think we are fortunate to have a progressive government and public support to take action rather than squabble like a group of spoiled children, although America does resemble a restless teenager socially.

I had a chance to read both papers, and for starters, I found both your and Elizabeth's papers of great quality and clarity.Very easy to follow the pionts being made and methods being used, even for a layman like myself,

Let me say that it is refreshing to witness an actual scientific debate between actual scientists on the influence of a warming Arctic on NH weather patterns, and done so in the peer-reviewed scientific literature.

I found it interesting that there were many common findings between the two papers, such as reduced wind speed and wave velocity in fall, which strongly suggests that additional merit in your hypothesis of a jet-steam link to Arctic amplification.

Also, it was interesting to read that Elizabeth found an increase in altitude of the large 500mb waves, consistent with gradual atmospheric warming, but did not find increased amplitude with the waviest contours. Kind of like the whole Rossby wave pattern is lifted up but not moving closer or further from the pole.

The main disagreement seems to be on the "amplitude" of the waves and if they are changing or not. But the amplitude may not be the biggest issue for the effects on the surface ? After all, if the amplitude of these waves stays the same, but the pattern is blocked, then we still will have prolonged extreme weather events.

Seem that for example here in California, it's been bone dry over the raining season, apparently because of a blocking high that's been sitting in the North Pacific off the Washington coast for, what is it, almost a year now ?Now, I don't think that particular high has much to do with Arctic amplification, and Barnes 2013 did not find an increase in 'blocking' events.

But blocking events, the jet-stream and the relation to Arctic's changing climate are often discussed at Neven's Arctic Sea Ice blog (where I'm a regular), and I'm very much interested in presenting your list of papers there and open a discussion at Neven's.

Rob Dekker, You could not have read these papers very carefully. Barnes' paper effectively disproves the claims of the Francis and Varvus paper, which claims that blocking situations with strong cold-air advection have gotten more frequent due to a weakening of the jet stream. And the fact that temperatures do not show such an increase in cold events confirms Barnes' findings...cliff

In response to Ms. Francis defense of her paper countering Barnes via Mass in this blog post thread, while I have not yet read the Barnes paper, Ms. Francis's recent work has been cited several times in recent years by some meteorologists who have taken a liking to it, such as Jeff Masters at the Weather Underground (now owned by The Weather Channel which itself is owned by GE's NBC, et al and let us not forget GE's NBC is in the media business and they will go all out to capture the attention spans of people in order to sell advertisements).

With that said, good scientists should be careful not to have love affairs with their own or others hypotheses. Good scientists should be skeptical (note: skepticism does not equate to cynicism). The challenge I see for Ms. Francis, et al hypothesis is this notion of blocking patterns and "stuck" / often "extreme" weather. The media circus about the "polar vortex" doesn't actually help Ms. Francis support her case from the perspective of most common people, because this recent "polar vortex" is in fact not hanging around very long. It sure doesn't look like its being blocked or part of a blocking pattern. So it looks like a tough sell to most people who don't have enough educational foundation of synoptic and dynamic meteorology.

Jennifer, A strong contradiction to your theory is the fact there is no evidence of increased cold waves during the past decade, which you wold expect from increased amplification. Can you provide any evidence of such a reflection in the thermal fields at the surface or 850 hPa? And as you probably know, Elizabeth Barnes had completed a new paper documenting that the amplitude of blocking has not increased. Furthermore, the ice/snow field has been dropping for decades over the arctic. Why haven't we seen the cold waves increasing before. Thus, both direct and indirect evidence suggest that your hypothesis is not correct..It did stir up a lot of discussion, which is good...cliff

I don't see how you can look at the second-to-last graph, the one showing the plot of "the % of the country with a extreme low minimum temperatures" and say that, on average, winters aren't getting warmer.

First, I never said that this week’s cold spell was related to climate change. In fact, when asked this question by numerous reporters, I have said “no” consistently and emphatically. Second, I agree with you that record warm temperatures have been broken much more often in recent decades than have record cold temperatures. This makes all kinds of sense as the globe warms overall, and especially as Arctic airmasses warm even faster than the global average. This is NOT what our hypothesis addresses. What we are suggesting is that in response to a relaxed poleward temperature gradient, the jet stream’s trajectory will become more amplified, which will cause more frequent excursions of cold air southward and warm air poleward. The air within those troughs will be warmer on average, and the warm air in the ridges will be also warmer. The duration of these anomalous excursions will also likely increase, as we know that waves in amplified jet-stream patterns tends to progress eastward more slowly, and thus the weather they create persists for longer periods.

Third: Regarding Libby Barnes paper, did you read my earlier comment about her claims that our metrics were flawed? And did you notice that the results in that same paper support our hypothesis, namely that wave propagation speed is decreasing significantly in fall (statistically speaking) and also that amplitudes are increasing? Her claim that we’re using the wrong height contour just does not make sense – the height contour we used rests squarely in the path of the jet stream and tracks its trajectory, while the “waviest” contour she identifies is well north and tracks a very different wave character. She’s a very talented dynamicist and communicator, but on this particular point, I respectfully disagree with her assessment. Please check again.

Fourth: I have read all of her work showing convincingly that blocking highs are not increasing in the real world and are projected by climate models to decrease under global warming. What I see in the reanalysis data, however, is that most extreme weather events associated with persistent weather patterns (e.g., extremely snowy winter in AK during 11/12, flooding in Spain in 9/12, very stormy pattern in the UK last month, very warm winter so far in AK, very warm winter in the eastern US in 2012, floods in Alberta, just to name a very few) are NOT associated with blocking highs, but rather are just a highly amplified jet trajectory or a cut-off low. This type of pattern IS increasing over time. What’s not clear is whether the models adequately capture these patterns.

Finally: I have stated a zillion times that this line of research is in its infancy, and we have so much more to learn. I am thrilled that so many excellent minds are focusing on this topic, so I’m confident that progress in attribution and mechanisms will be made in short order.

We are on the same side here. I keep coming back to the fact that 75% of Arctic sea ice has been lost and the region is warming 2-3 times faster than the mid-latitudes. This MUST have an impact on the large- scale circulation system. What I see is the pile of evidence supporting our hypothesis is growing, while the pile contradicting it has nothing in it. Honest skepticism is welcome, as the jury is clearly still out.

A warming arctic leading to a weaker jet stream with larger Rossby waves is a theory, right? Jeff Masters writes "the research linking climate change impacts in the Arctic to more extreme jet stream patterns is still very new, and we need several more years of data and additional research before we can be confident that this is occurring." So are you saying there is data showing that it is definitely not occurring? Theory disproven?

Ted, The first part of Holdren's message was fine--that one event proves nothing. But then he goes off saying that such cold waves are what we might expect from global warming. This is where he crossed the line into unwarranted (and scientifically unsupported) speculation. This is a boon to global warming deniers. On one hand the IPCC says cold waves will be less frequent (makes sense) and Holdren says they could be more frequent. Leaves me scratching my head..cliff

Jennifer, My point, and the results of Libby Barnes' research, is that there AREN'T increased daily excursions of the jet stream and height lines. Your work is being used by well-meaning folks to suggest that global warming will result in increased cold waves. Libby's work shows that increased excursions are not happening and the surface temperature data shows that increased frequency of cold waves are not happening.

As I noted before, the claims of increased cold waves under global warming have been a field day for those who believe GW is unfounded. And it is directly opposite to what is claimed by the IPCC. ..cliff

Ted, I understand you are unhappy with me criticizing Holdren, but I really think his video was highly counterproductive. This is very weak speculative science and only provides a large target for skeptic groups that can easily show why it doesn't make sense...cliff

Your points about Libby Barnes’ 2013 paper are correct *only if* her claim that the 500 hPa contours we used for our analysis were incorrect, a claim that I respectfully dispute for reasons I stated in an earlier comment. Using the contour that best tracks the path of the jet stream, rather than the waviest contour that occurs well north of the jet stream (she compared analyses using both in her paper), then she *does* find increased wave amplitudes (see gray bands in her Fig. 3), she *does* find significantly slower wave propagation speeds (her Fig. 4b), and she *does* find slower zonal wind speeds (her Fig. 4a), all of which agree with our results and support our hypothesis.

Regarding the possible increase of cold waves in response to a wavier jet stream, it must be made clear what is meant by a cold wave. I was *not* referring to record-low-temperature-breaking cold extremes. I am referring to events of below-normal temperatures, which occur in deep jet-stream troughs. We know that “normal” is based on the mean over the previous 3 decades, so as the climate warms, the mean with which anomalies are calculated is also warming. The threshold for a “cold wave,” consequently (say, 1 standard deviation below the mean), will also rise, and the temperatures in a future cold wave will be warmer than in one today, on average. But it will still feel relatively cold, as nicely explained in Seth Bornenstein’s piece last week.

Jennifer, I can't agree with you on this. Libby Barnes pointed out an essential methodological problem with your paper: the fact you based you evaluation of the amplitude of the waves NOT with instantaneous amplitude (the amplitude of waves at a particular time, which is what really counts in a weather situation), but based on seasonal excursions, which means nothing for individual cold waves. She demonstrated that the correct and seasonal methods give different answers. Thus, the conclusions in your paper about a wavier jet stream are problematic. And I should note that temperatures on the ground agree with Libby..there is NO evidence of increased cold waves as the polar areas warm. You have to agree with that...cliff

I know little of science but I do know how the media can distort a story. I have been looking for an objective website on global warming.

I don't trust the media. I know that winters seem to be less severe than they were when I was a kid. I grew up on a farm and have a tendency to watch the weather more closely than some people.

I remember very well the Global Cooling scare of the 70's which they announced during a severe winter.

I also remember the media announced we were having Global Warming during the summer of 1988 when we had a severe drought and heat wave all across the USA.

So folks, I hope you'll pardon me for not jumping on either bandwagon. The report that some GW advocates faked some statistics does not help. On the other hand I've seen where deniers fake stats too.

I tend to believe in the warming scenario but I have an extreme dislike for those who smugly proclaim we are dumb if we don't believe in it.

As I see it the main stream media has done a lot of damage to those who believe in global warming. They have done it by assuming that the masses are stupid and must be scared into believing. They make it sound like GW will happen tomorrow and we are all going to die. However, we can stop global warming only if we listen to them. Any voices of dissent are stupid and must be mocked.

There is one site that says in order to prevent GW all non-believers should be killed. I don't want people like that ruling over me.

People don't realize it but Russia and China would really like to see the USA fully adopt all measures to prevent Global Warming. That's because it would leave us in a less strong position industrially and militarily. Go read the English version of Pravda if you do not believe me. These two countries have no intention of following any standard that might weaken them. They are not above using the internet to influence Americans.

I just want to know the truth either way. If we have to change how we live then everybody better change and not just this country. I also don't want to see the 'elites' living it up while I'm shivering in a cave with everybody else.

No I don't believe I'll be living in a cave but I have actually seen the warming people say that several families must share one car. Also we must get rid of all our pets because even a small dog like my dachshund puts out the pollution of three SUV's yearly.

In closing I just want to say that I don't trust the motives of many organizations involved in this issue be they warmists or deniers.

Something that I am concerned with is how everyone around me has claimed that all this cold weather disproves global warming to the T. I have political friends on Facebook that actually get angry about the thought of global warming and how all this weather proves that global warming scientists are just blowing hot air into the ears of government officials.

The fact is, just like this article implies, this "extreme" weather does not prove or disprove global warming. Weather varies. It happens. Sometimes it is colder than previous years. On the other hand, has anyone actually seen the documentary An Inconvenient Truth? Al Gore has predicted this stuff. His research and contacts have predicted these 'extreme' weather patterns, these 'polar vortexes,' and other weather phenomenon that has been happening for the the last five years.

Extreme weather has been seen across the globe, ocean temperatures have risen, polar ice caps are melting, tornadoes are getting worst across the United States, extreme drought and heat is present in the summer months, yet people, especially political figures, are still arguing whether any of it really exists. Ask your grandparents and great grandparents how the weather was when they were a kid. There is a huge difference today compared to the weather fifty years ago. Yes, weather varies, but usually it takes thousands of years for weather patterns to change and shift as much as they have just in the last fifty years. Things are changing, and I still cannot believe that people are denying that. It is happening before our eyes. Open your eyes people! Open them up and quit denying what is going on around you!

Cold and ice are mankind's enemy. At no time in geologic history has the earth's land area been too hot for life to survive. It has been too cold though. Nothing can grow on ice, it is a death sentence for mankind. A gradually warming planet is a blessing. It turns glaciers it no alpine valleys where Heidi can tend her cows. It pushes the tree line north where nothing but lichens can grow today. Even polar bears seem to be able to change their diets ( they are bears after all ) and can eat caribou or other large grazing animals.Read more at http://globaleconomicanalysis.blogspot.com/2014/01/europe-dumps-global-warming-efforts.html#qb0m0gSW1Jjslwkd.99global warmingdrug addiction

Nice work, except for the claim that warming is going to resume: "Global warming will occur over the coming century and only can be determined statistically since there is a lot of natural variability in the climate/weather system. One event proves nothing. Furthermore, the real warming is in the future."

All the evidence is that the "real warming" has ended, because all the evidence says that the bulk of 20th century warming was caused, not by CO2, but by the 80 year "grand maximum" of solar activity that began in the 1920's. Almost everyone agrees that CO2 has a very modest forcing effect. For this forcing effect to explain more than a small amount of 20th century warming it has to be multiplied up several times by water vapor feedback effects and there is absolutely NO evidence that water vapor feedback effects are even positive.

That's the fraud aspect of the IPCC. Their "estimates" of the water vapor feedback (climate sensitivity) are derived by ASSUMING that virtually all 20th century warming was caused by CO2. To be specific, solar variation is assumed to have 1/56th the forcing effect of increases in CO2: .05 w/m^2 vs 2.83 w/m^2. That is from the radiative forcing chart on p.696 of AR5:

This is on the INPUT side of the models, which then just take the amount of observed warming and calculate how sensitive to CO2 the climate would have to be in order for the warming to have been caused by virtually entirely CO2, in accordance with their assumptions.

Sorry but the ASSUMPTION that warming was caused by CO2 cannot provide EVIDENCE that warming was caused by CO2 (that sensitivity is high).

The assumption that temperatures have been CO2 driven is the reason why sensitivity estimates have been dropping precipitously in the wake of the 15+ year "pause" in global warming. As the rate of warming that is being presumptively explained by CO2 falls the implied warming effect of CO2 falls accordingly. But these estimates are still FAR too high because the real implication of "the pause" is that late-20th century warming was NOT caused by CO2 but by the solar turn from grand maximum to deep minimum. That would leave little warming left to be explained by CO2, implying that water vapor feedback effects are either close to zero or possibly even negative (as cloud formation induced by increased evaporation both serves as an effective heat pump from the surface to the upper atmosphere, and serves to block incoming solar radiation).

That's what the ACTUAL evidence points to, both in the paleo data (lots of paleo correlation between solar activity and temperature but a discernible paleo correlation between CO2 and temperature only in the temp-causes-CO2 direction), and in current observations (the absence of the upper-tropospheric "hotspot" fingerprint of positive water-vapor feedbacks and other missing "fingerprints"). So no, if you follow the actual science instead of the IPCC psuedo-science, if you follow the EVIDENCE, the best estimate is that we are now at the peak of the modern warm period, with cooling coming next.